[FSDP] Stabilize fully_shard overlap timing test

[albmalamd]

The overlap test emulates compute and communication with fixed-cycle GPU sleeps. DVFS and device contention can change their wall-clock duration between separate single-shot measurements, causing false failures under load.

Interleave reference and test measurements and compare their least-perturbed samples. This only improves timing stability; it does not relax assertions or tolerances, change emulated sleep durations, or add conditional retries or skips.

Test Plan:

python -m pytest test/distributed/_composable/fsdp/test_fully_shard_overlap.py::TestFullyShardOverlap::test_fully_shard_training_overlap

Disappeared timing assertion flakiness

Motivation

Technical Details

Test Plan

Test Result

Submission Checklist

• Look over the contributing guidelines at https://github.com/ROCm/ROCm/blob/develop/CONTRIBUTING.md#pull-requests.